1. Field of the Invention
The present application relates to the technological sector of the component parts of a hip prosthesis.
2. Description of the Related Art
As is known to those skilled in the art, these component parts consist of a stem, which is fitted into the cavity of the femur, and into this stem is inserted a “neck” (referred to as mobile in the sense that it does not form an integral whole with the said stem), fitted to the free end of which is a spherical head designed to mate with the cavity of the acetabulum.
Mobile necks of the type described above were devised to make it possible, as the need arose, by changing the type of neck used, to create a prosthesis that is anatomically and ergonomically suitable for the skeleton of the patient being fitted with a hip prosthesis.
Mobile necks are therefore produced in sets so that their various dimensions can be used to cover all possible cases. In these sets each neck has a different length and inclination, in such a way that, depending on the physical structure of the patient, the prosthesis is suitable for that structure. In an intervention to fit a hip prosthesis, two important dimensions must be observed: the so-called off-set, which is the distance between the axis of the stem and the centre of rotation of the head, and the height, meaning the distance between a plane perpendicular to the axis of the stem where it connects with the neck, and the abovementioned centre of rotation of the head.
A typical example of the related art is that of Schelhas (U.S. Pat. No. 4,822,370). Schelhas pertains to a hip joint femoral prosthesis having a shaft, a spherical head and connecting part between the shaft and the head. However, Schelhas offers only the possibility of acting on the CCD angle. Thus, the prosthesis of Schelhas allows to position the prosthesis with a certain “tension” of the involved muscles (on the transverse plane) However, if the surgeon rotates (in the recess of the stem of the prosthesis) the neck about its axis to optimize the “tension” of the muscles also varies the position of the center of rotation on the sagittal plane. On the front plane Schelhas does not offer any possibility of variation of the length of the leg of the patient. On the sagittal plane it is possible to vary the stability of the neck but at the same time the “tension” of the involved muscles is also varied. In other words, the angles of antroversion and retroversion in this related art of Schelhas are not independent from one another and the variation of one angle also affects the other angle.
As a consequence, with the sets of mobile necks currently existing on the market, since it is only possible to modify the length or the inclination, the ideal value is obtained for usually only one of these two dimensions, that is either the offset or the height. The reason for this, of course, is that the desired offset can be obtained by for example reducing the length of the mobile neck, but this would simultaneously reduce the height in a way which might not always be acceptable if the intervention is to be successful.
On the other hand there is no question of supplying “made-to-measure” mobile necks in the sense of having the right length and the right inclination relative to the stem, because in the first place the exact measurements can only be found out during the intervention, and in any case the costs would be prohibitive, precluding many people from the chance of undergoing an intervention to fit a hip prosthesis.